There is an increasing demand for the management of high currents in a very small space and in harsh environments exposed to large temperature changes. Thus, in the automotive sector, due to increasing electrification functions, the high current demand increases enormously as for inverter and E-motor drives in hybrid car applications, starter-generator applications, and high power DC/DC converters or x-by-wire applications used for electric power steering or electric braking. These applications have high current carrying requirements in a minimum space, challenging state-of the art power modules in terms of achievable power density.
In order to provide sufficiently high power energy sources, the typical 12V automotive battery power net would require currents of about one hundred volts up to thousands amperes. Thus a 42V power net has been employed to provide higher power while keeping the current at reasonable values, in the range of 100 amperes in order to reduce cable thickness and conduction losses (I2R).
Hybrid electric vehicles (“HEV”) have now entered the automotive market. Equipped with huge electric motors such cars can be operated without the combustion engine. These E-motors are typically in the power range of 15 kW to over 100 kW. Correspondingly the voltage of the power source driving these motors had to be increased beyond the HV net. Then, even 42V was not enough to keep the phase currents of those high power motors and applications to a reasonable order of magnitude.
Therefore, HEVs are equipped with a so called dual power net or dual bus voltage system. Conventional electric systems are still powered by the 12V battery. However, in order to power the E-Motors and power intensive auxiliary systems a second very high voltage battery has been introduced. This is normally a NiMH or a Li-Ion battery with output voltages of from 150V to over 600V with good energy storage capabilities. Instead of having a 600V battery, a lower voltage battery, e.g. 150-300V can be boosted to 600V or greater via a DC/DC converter.
The existence of a high voltage system in HEVs causes a major challenge: the 12V powered systems need to be well protected from the high voltage system since standard automotive blocking and protection circuits are not dimensioned to withstand more than 30V or 40V. Even the known 42V system specifications do not allow more than 60V in the entire power net due to safety reasons.
With a high voltage source in the car the low voltage (LV) and high voltage (HV) power supplies need to be very well isolated from each other. Otherwise the LV components would immediately be destroyed by accidental direct contact to the HV net. In addition to the electrical damage the HV bus also needs to be secured against accidental contact by humans. Thus, the 12V battery was relatively safe and not life threatening; an accidental contact to the new HV power net can be deadly. Therefore, the HV power circuit needs to be 100% safe and protected from any accidental contact by humans. Correspondingly it must be assured that the LV power net, which can normally be touched, is well isolated from the dangerous HV circuit.
This isolation problem between the LV and HV bus is quite complicated since the two electric circuits in a car still need to communicate with each other. For example all the microcontrollers and other control elements such as Electric Control Units (ECUs) are integrated in the 12V power net. These microcontrollers and computers determine the various drive modes and control the entire behavior of the overall system. Therefore these microcontrollers need to send commands to the HV control electronics. Specifically, the HV gate driver integrated circuits (ICs) which control the high power motor control electronics need to exchange information and commands from the LV electronics.
FIG. 1 shows a typical E motor drive system for E motor 30. A low voltage microcontroller 32 (and its ECU—not shown) is powered by low voltage battery 31 (12 volts) and controller 32 is coupled to the drivers of a high voltage inverter consisting of three half bridge IGBTs within dotted box 33. A high voltage battery 40 (600 volts) and filter capacitor 41 powers inverter 33. Each IGBT has a suitable IC driver, shown as high side drivers 34, 35 and 36, and low side drivers 37, 38 and 39 for E motor phases U, V and W. The low voltage system microcontroller 32 must be coupled to the high voltage side of the system to drive as schematically shown by the lines labeled “control signals”.
As previously stated, it is necessary to isolate the LV network (shown with the microcontroller and the ECU in block diagrammatic form) from the HV circuit. FIG. 2 schematically illustrates a prior art type of isolation barrier 50, known for such circuits. Typically the isolation should be capable of insulating 2 times the HV bus voltage plus 1 kV. (2200 volts in the case of FIG. 2.) With typical HV bus voltages of greater than about 500 volts, the isolation will be in the range of greater than 2 kV to 4 kV. It has been very difficult to establish such a high isolation which is reliable and rugged enough for harsh automotive conditions.
In FIG. 2, the low voltage system to the left of barrier 50 is powered by the battery 31 in FIG. 1 and includes the mixed signal controller 49 and a related electronic control circuit. Microprocessor 32 and its ECU 42 have the conventional functions as indicated in FIG. 2. Thus, digital outputs to the high side and low side drivers are taken through barrier 50. Current sense resistor 51 is also coupled through barrier 50 and resolver 52 is coupled back through barrier 50 to the digital inputs in the LV microcontroller 50. A temperature sense device 53 in the HV side of the system is similarly coupled back to a temperature sensing input in L.V. control 49.
These connections through the isolation barrier 50 conventionally employ opto-couplers, capacitive couplers, inductive couplers and/or transformers. The signals are exchanged via these couplers which have to provide the kV isolation.
The main disadvantage of such isolation is that each signal connection must be separately isolated.
Therefore, just the gate driver signals for the full bridge inverter requires 6 couplers (3 high side and 3 low side). The sensor signals indicated in FIG. 2 require even more couplers.
Major disadvantages of prior art solution are the cost and the space requirements for those couplers.
In addition, those components must be very reliable, require very expensive components to withstand the typical “under the hood” requirements of an automotive application over its lifetime. Also, degradation of the isolation layers (e.g. in capacitive couplers) and the degrading in performance of the optics of the opto-couplers is a major concern and the automotive industry has been reluctant to use them. Transformers are, therefore, often preferred but are an expensive and space consuming choice. Therefore it is desirable to find an alternative solution to those prior art couplers.
In order to address some of the above mentioned problems with the signal isolation it is also known to provide on-chip isolation between the HV and LV sections of a single IC. For example, a solution called “core-less transformer on chip” is known. The gate driver IC of that solution provides a transformer structure on a chip, established by integrating metal coils in the semiconductor process. Due to the near proximity of the two transformer coils on one IC no classical “core” is used. The isolation layer is provided by an isolating structure of the semiconductor process, e.g. a nitride or oxide layer. The disadvantage of this coreless-transformer-IC solution is that the kV-isolation fully depends on the integrity and ruggedness of the IC process. Any defect of the isolating layer will connect the LV to the HV section of the power net. Therefore, reliability is a serious concern for this solution. In addition the core-less-transformer structure requires a lot of space in order to guarantee greater than 2 kV isolation. The increased silicon chip area also increases the cost of such a gate driver IC beyond that of the area needed for the gate driver function only.